Many different types of material are comminuted, i.e., the size of the material's particulates are reduced, for a variety of different reasons. For example, coal excavated from a mine is frequently comminuted to make the particulate size smaller and more uniform to facilitate the coal's transportion and/or to provide consistent combustion in a furnance. Food stuffs, such as wheat, are frequently comminuted to produce flour. And rock containing a desirable ore is frequently comminuted to provide easier access to the ore and the metal included in the ore.
A common way of comminuting material is to collide a particle of the material with an impact surface. The collision generates a force on and inside the particle that causes the particle to fracture into two or more smaller pieces. The amount of force generated in the collision is directly proportional to the impact speed of the particle—the speed of the particle relative to the impact surface at the moment of collision—and increases as the impact speed increases. As the amount of force generated on and inside the particle increases, the size of the pieces that result from the collision of the particle with the impact surface decreases.
There are many different comminuting devices that collide a particle of material with an impact surface. For example, Hammer mills comminute particles of material with a rotating set of hammers having impact surfaces. In operation, the material is dropped into the mill and fed by gravity to the hammers. The hammers smash the particles of the material into smaller pieces and also throw some of the particles and pieces against a side of the mill. In a hammer mill the impact speed of the particles largely depends on the rotational speed of the hammers.
Another type of comminuting device is a pin mill. The pin mill comminutes particles of material with multiple rings of pins spinning in opposite directions. In operation, the material is dropped into the center of the mill and moves outward through the paths of the pins in each ring. As the particles of material move, the pins knock the particles. In a pin mill, the impact speed of the particles largely depends on the speed of the pins moving along the paths.
Another type of comminuting device is a jet mill. Jet mills comminute particles by accelerating the particles with a jet of air and directing the accelerated particles against an impact surface, which may or may not be stationary, or against an opposing jet of particles. In operation, a jet of air is generated and the particle is then fed into the jet to accelerate it. Once accelerated to a desired speed, the particle is directed toward and collides with the impact surface or another particle of an opposing jet. In a jet mill, when the impact surface is stationary, the impact speed of a particle largely depends on the speed of the particle, and when the impact surface moves, or an opposing jet of particles is used, the impact speed of a particle largely depends on the combined speed of the particle and the impact surface or particle of the opposing jet.
Unfortunately, each of these comminuting devices has some problems. Each of these devices is not very efficient for comminuting many types of material, i.e., a comparison of the amount of energy these devices consume to comminute a material with the value of the material at a given particulate size. Each comminuting device consumes a substantial amount of energy to comminute a material to a desired particulate size. Because hammer and pin mills typically generate a maximum impact speed of about 350 ft/sec compared to an impact speed of about 550 ft/sec or more, which is typically desired for efficient comminution, as indicated in tests, a significant reduction in a material's particulate size typically requires the material to be run through these mills more than once. Thus, the amount of energy consumed during the comminuting process includes the amount of energy required to operate these mills during multiple runs. Furthermore, to generate a higher impact speed (greater than about 550 ft/sec), the hammers and pins would have to rotate/move faster than their conventional structures will allow without sustaining substantial wear or catastrophic failure. Although jet mills can generate higher impact speeds than hammer and pin mills, the amount of energy jet mills consume can also be significant because they generate a jet of air to accelerate a particle, which typically requires a substantial amount of energy.